Sense loading

Figure 9.5 shows a main servo controlled by two thermostatic pilots sensing load temperature (type CVT) and a solenoid valve (type EMV). Any of these pilots may be used separately with the servo valve. 

By locating T-sensors as near to the loads as possible to assure that they are sensing load temperatures, not furnace temperatures. 

Capacity Control The simplest way to regulate the capacity of most steam vacuum refrigeration systems is to furnish several primary boosters in parallel and operate only those required to handle the heat load. It is not uncommon to have as many as four main boosters on larger units for capacity variation. A simple automatic on-off type of control may be used for this purpose. By sensing the chilled-water temperature leaving the flash tank, a controller can turn steam on and off to each ejector as required. 

Of special interest in scale-control systems is the type in which the motion of the scale beam is sensed by a differential transformer or a group of load cells. The output of such devices is proportional to the displacement of the scale beam, which in turn is proportional to the amount of material in the weigh bucket. Many designs use loadsensing devices such as strain gauges or transducers. These eliminate the need for a scale-beam mechanism. The weigh vessel is mounted directly on the load-sensing devices. This provides many benefits in... 

The NPDES regulations (40 CFR 122.45) require effluent limits to be expressed as total recoverable metal. This requirement makes sense as a means to monitor and regulate both the total metal loading and also the effectiveness of wastewater treatment that involves chemical precipitation of the metal. 

Speed-Control Systems The most common sensing element is mechanical some systems are hydraulic or electronic. For valve positioner they all have a hydrauhc servo as first choice, with an occasional choice of pneumatic for lighter loads. 

Various protective instruments are used to provide a shutdown signal (to a fast-acting trip valve at the expander inlet) that senses various things, such as overspeed, lubricant pressure, bearing temperature, lubricant temperature, shaft runout, icing, lubricant level, thrustbearing load, and process variables such as sensitive temperatures, levels, pressures, etc. However, too many safety shutdown devices may lead to excessive nuisance shutdowns. 

For automatic control of the couplings, sensing devices may be placed in the flow circuit of pumps, fans or any drives that have variable load demands... 

To ensure proper sensing of the incoming current and its phase displacement by the relay it i.s es.senlial that the CTs ratio and their VA burden chosen for the required duty are close to the actual requirement as noted in Table I. S.8.. Sometimes this fact is overlooked and CTs with a much higher VA burden or ratio or both are chosen while the secondary circuit may not be adequately loaded. In this case the CTs may not accurately transform the primary parameters to the secondary and. in turn, the relay may not send accurate signals. Moreover, the relay itself may operate only at minimum 19c or more of its rated current (I or 5 A), depending upon its design and type (lEC 60051-1). 

Shock-compression processes are encountered when material bodies are subjected to rapid impulsive loading, whose time of load application is short compared to the time for the body to respond inertially. The inertial responses are stress pulses propagating through the body to communicate the presence of loads to interior points. In our everyday experience, such loadings are the result of impact or explosion. To the untrained observer, such events evoke an image of utter chaos and confusion. Nevertheless, what is experienced by the human senses are the rigid-body effects the time and pressure resolution are not sufficient to sense the wave phenomena. 

This indieates the poor intrinsie eross-regulation eapabilities of the transformer, whieh ean be slightly improved by using filar winding teehniques deseribed in Seetion 3.5.9. If the -i-5 V and the -i-I2 V outputs were both sensed, and the -i-5 V output was then loaded as deseribed previously, then the -i-I2 goes to -tI2.25 V and the -12 V goes to -12.75 V. 

The eenter node of the resistor divider beeomes a eurrent summing node where a portion of the total sense eurrent eomes from eaeh of the sensed output voltages. The higher power output, and usually the output that requires a tighter output regulation, require the majority of sense eurrent. The lighter loaded... 

Let us again examine the power supply with +5 V, +f2 V, and -12 V outputs. Sinee +/-12V loads usually provide power to op amplifiers that are relatively immune to variations on their Vee and VEE supply lines, their voltage regulation ean be looser. Using the same situation as the first example in this seetion, Ri is 2.49 K and a sense eurrent of 1.004 mA. 

Improvements in all eombinations of loading are experieneed when the outputs are multiple sensed. 

In order to enhance the cross-regulation of the multiple outputs, it is desired to sense a portion of all the positive output voltages. To do this, one must investigate the technology of the circuits that must draw their power from the outputs. The hypothetical loads, in this case, are... 

To improve the effeets of poor output eross-regulation, sensing some eurrent from eaeh positive output greatly improves the behavior of eaeh output in response to ehanges in individual loads. 

Every current-mode control application that exceeds 50 percent duty cycle must have slope compensation on the current ramp waveform. Otherwise an instability will occur whenever the duty cycle exceeds 50 percent. This is typically done by summing into the current waveform some of the oscillator ramp waveform. This will increase the slope of the current waveform and therefore trip the current sense comparator earlier. A common problem is the inadvertent loading of the oscillator, so I will use a PNP emitter-follower to buffer the oscillator. The circuit configuration can be seen in Figure 3-74. 

For centrifugal and axial compressors, some form of override control is recommended for constant speed motor drivers to sense motor over load and override the process control until the cause of overload has... 

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